The present invention relates to a direct-current (DC) power supply system which generates stable DC power of high-voltage and high-output performance after three-phase bridge-rectifying, and more particularly, to a DC power system adopting a three-phase inverter by which a modified waveform for an output control is produced. There are many types of DC power supply such as slidac-adopted, thyristor phase controlled, linear and switch mode power supply etc. As a conventional high-voltage and high-power DC power supply system, there are a slidac-adopted output control type and a thyristor phase controlled output control type, respectively as shown in FIGS. 1A and 1B.
However, according to such conventional methods, though the slidac-adopted output control type shown in FIG. 1A has the same characteristics throughout the range of all outputs, it is not easy to control automatically the slidac for the output control, and it is difficult to make the slidac for high power system. Also, since the thyristor phase controlled output control type shown in FIG. 1B controls the output by a phase control of the thyristor, an automatic control is easy to be attained and the power converting efficiency is also high. However, homogeneous characteristics cannot be obtained throughout the range of all outputs. Particularly, during low-voltage output voltage ripples increase such as sawtooth wave.
To be described in more detail, the characteristics of the output voltage waveform of the conventional apparatus shown in FIGS. 1A and 1B are shown in FIGS. 2A to 2D, in which FIG. 2B shows the output voltage waveform for a slidac-adopted output control type, FIG. 2C shows the whole output voltage waveform for a thyristor phase-controlled output control type, and FIG. 2D shows the low output voltage waveform for the thyristor phase-controlled output control type.
In the conventional 3-phase power rectifying power supply circuit, if the input waveform is a normalized sinusoidal wave sin (wt) with respect to the 3-phase bridge rectification, the maximum value of the output voltage is .sqroot.3 (.apprxeq.1.732), and the minimum value thereof is 1.5 for both cases of using slidac and thyristor, as shown in FIG. 2A to 2D. Thus, the ripple voltage of the maximum output is 0.232 for a circuit having no smoothing filter such as inductor or capacitor. Therefore, the conventional apparatus involves a problem of the voltage ripple.